Why is this the answer?

The answer is leaves because this clever riddle uses a wonderful word picture to describe the life cycle of a plant's greatest treasures. Let's look closely at the first two lines: "They're up near the sky, on something very tall." When you are walking outside and look way up high, what do you see covering the big, tall branches of a tree? You see the beautiful green, red, or yellow parts of the tree. These little parts are the leaves, and they are usually way up high, making them seem close to the sky. A tree is certainly something very tall, and it is the special highway that carries the leaves that high. The leaves love to sit on the branches, soaking up the sunshine so they can make food for the rest of the tree. They start to grow when the weather warms up in the spring and spend all summer long working hard for the plant. Now let's look at the next line, which is the trickiest part of the riddle: "Sometimes they die, only then do they fall." When we talk about leaves "dying," we are really talking about what happens to many trees when the weather starts to get cold in the autumn and winter. The tree is getting ready for a long winter nap. When the chilly air arrives, the leaves change their color from being a bright, healthy green to beautiful colors like bright red, glowing gold, and warm brown. This color change is a sign that they are finishing their work for the year and are not needed by the tree anymore. When a leaf turns its final color and becomes brown and crisp, it means it is completely ready to drop off the branch and float down. The tree gently lets the leaves go, and they flutter all the way down to the ground. That is why the riddle says, "only then do they fall." They stay safely on the tall tree all spring and summer, no matter how windy it gets. Only when they have finished their job for the season do they finally fall down to the ground for you to crunch through on a cold day! This whole riddle is a sweet puzzle all about the amazing and colorful journey of a leaf on a tall tree.